Drilling for hydrocarbon production, particularly gas, in shale formations often involves drilling a horizontal section in the well. In one technique, after the casing has been cemented, the operator runs a bridge plug and perforating gun on tubing into the horizontal part of the well. The operator sets the bridge plug and perforates the casing above the perforation. The operator may then retrieve the tubing and perforating gun and hydraulically fracture or “frac” the well by pumping fluid and proppants down the casing string and out the perforations. After the first perforation, the operator may run another bridge plug and perforation gun on a tubing string and repeats the frac operation. A number of zones may be perforated and tracked in this manner. Alternately, the operator may make a single run on tubing with a number of perforating guns and bridge plugs, and stage the setting and fracking of the various zones. In that instance, the frac fluid is pumped through the tubing.
In both techniques either a drilling rig or a workover rig is required to run the tubing. Many gas wells are produced without tubing being installed, and many operators prefer to frac through casing, rather than through tubing. Consequently, the tubing may be used only to enable the bridge plugs and perforating guns to be conveyed into the horizontal portion of the well. Having a drilling or workover rig on site for the perforating and frac operations adds to the expense of the well.
Bridge plugs and perforating guns in general can and are frequently deployed with wireline, rather than on tubing. The wireline may be deployed from a winch mounted to a truck, without requiring a drilling rig or workover rig. In a vertical well, gravity pulls the bridge plug and perforating gun into the well. However, if the well has a horizontal section, additional assistance is needed. Tractors powered though the wireline may be incorporated with the bridge plug and perforating gun to pull the equipment along the horizontal part of the well. Tractors, however, can be expensive and troublesome. Another way is to pump the perforating gun and bridge plug into the horizontal section. As the bridge plug moves downward, it pushes displaced fluid in the casing in front of or below it. Normally, the cementing of the casing, however, creates a closed chamber, with no place for the displaced fluid to flow, unless the well has already been perforated.